Not Applicable
Charge pumps are known in the art. A charge pump comprises a series of charge transfer elements, each of which increase the output voltage by a quantity of approximately 1*Vin. The charges are integrated over complete clock cycles. Typically a resistive or capacitive voltage divider is provided at the output stage of the charge pump in order to sense the output of the charge pump. The resistive or capacitive voltage divider draws a certain amount of current, thus reducing the efficiency of the charge pump. It would be desirable to find a measurable quantity in the charge pump which is also related to output current in order to sense the output of the charge pump without applying a load to the output stage thereby improving the efficiency of the charge pump.
The presently disclosed method and apparatus provide the sensing of an output voltage of a charge pump without applying a load to the output stage. In the charge pump the voltage change which occurs across a capacitor of a stage of the charge pump when the charge pump transfers charge to the next stage is proportional to the difference between the voltage at the output of the charge pump under load, and the voltage which will be developed at the output of the charge pump with no load. There is an interval in the timing of the charge pump cycle after the first stage capacitor has transferred its charge to a second stage capacitor where the high side of the first stage capacitor has not yet been connected to the line voltage. During this interval the charge pump undervoltage detection circuit measures the voltage at the high side of the capacitor and compares this measured voltage to a reference voltage. When the voltage at the high side of the capacitor is below the reference voltage, it can be accurately inferred that the voltage at the output of the charge pump is more than a certain voltage below what the output voltage would be if it was unloaded. The amount that the output voltage is below its maximum value is directly related to the output current. In such a manner the output voltage of the charge pump is sensed without using a resistive or capacitive voltage divider thereby improving the efficiency of the charge pump.